docs: def clear(self, rows=None):
      """Reset episodes in the memory.
  
      Internally, this only sets their lengths to zero. The memory entries will
      be overridden by future calls to append() or replace().
  
      Args:
        rows: Episodes to clear, defaults to all.
  
      Returns:
        Operation.
      """
      rows = tf.range(self._capacity) if rows is None else rows
      assert rows.shape.ndims == 1
      return tf.scatter_update(self._length, rows, tf.zeros_like(rows))